Tube Sound

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Tube sound (or valve sound) is the characteristic sound traditionally associated with a Vacuum tube-based audio amplifier. All practical amplifiers distort to some degree; some audiophiles prefer the sound that is produced by the distortion characteristics of tube-based amplifiers. The audible significance of measurable effects on signals is a continuing debate among audiophiles.

History Before the commercial introduction of transistors in the 1950s, electronic amplifiers used vacuum tubes (a.k.a. "valves"). By the 1960s, solid state (electronics) (transistorized) amplification became more common, due to its smaller size, lighter weight, lower heat production, and improved reliability. However, tube amplifiers have retained a loyal following amongst some audiophiles, with some modern units commanding very high prices.

Some musicians performing on electric guitar, electric bass, and keyboards in a range of popular and jazz genres continue to use tube instrument amplifiers or preamplifiers.

Explanation Tube sound is present in two distinct fields:

Sound reproduction Some audiophiles argue that the sound - classically associated with tube amplifiers - is richer or warmer than the sound from typical transistor amplifiers, and for this reason more satisfying. Superficially, in sound reproduction systems, accurate reproduction of the sound of the original recording is usually the goal, ie "High Fidelity" (HiFi), and thus gross distortion is nominally a bad thing designers do not deliberately seek to introduce. However at the upper end of audio systems ("high end" or "audiophile" systems) it is debated whether "accuracy" can best be described by simply having a wide frequency response and low measured distortion levels or whether highest quality reproduction is that which sounds subjectively "natural" or "musical".

All real world designs distort to some degree, and designers seek a compromise in the nature of the distortion their design produces, that "sounds good", rather than that which simply measures well. In the high end world audiophiles will typically pay greater regard to how it sounds than to how it measures, to the point of just not caring about measured distortion levels at all. The reasons behind the "tube sound", and why some consider this to be better than the "transistor sound" are complex but discussed below.

Musical instrument amplification Some musicians also prefer the distortion (guitar) characteristics of tubes over transistors for electric guitar, bass guitar, and other instrument amplifiers. In this case, generating deliberate (and sometimes considerable, in the case of electric guitars) audible distortion is usually a specific goal.

Solid-state emulation The term can also be used to describe the sound created by specially-designed transistor amplifiers or digital modelling devices that emulate the characteristics of the tube sound.

Probable causes The "warmth" and "richness" typically associated with "tube sound" is due to significant levels of 2nd order distortion, typically coming from a single ended (classic tube class A amp design ) stage, often the output stage.

However the monotonically reducing purely harmonic distortion spectrum of the simple SE gain stage is not of itself anything to do with using a tube, and a similar distortion spectrum and sound can be obtained using the same circuit topology with , for example, a MOSFET.

Negative feedback Commodity transistor amplifiers, especially from the 1980s, typically used large amounts of negative feedback. This allowed a very low measure of total harmonic distortion. Typically tube amplifiers have little negative feedback applied, and in recent years there has been a tendency in transistor amplifiers to use less feedback and design more linear stages which has again narrowed the sonic differences between tube and transistor designs.

Audible differences Some audiophiles prefer the sound of tubes over transistors. As above this is in reality partly a function of the circuit topologies typically used with tubes vs the circuit topologies typically used with transistors, as much as the gain devices themselves. But there are also real differences. Although there are also similarities between say the characteristics of a triode and mosfet, or a tetrode and a bipolar transistor

Some sonic qualities are easy to explain objectively based on an analysis of the distortion characteristics of the gain device and/or the circuit topology. For example the triode SE gain stage produced a stereotypical monotonically decaying harmonic distortion spectrum that is dominated by significant second order harmonics that made the sound seem rich or even "fat".

However, other audible differences in sound have proven difficult to define or measure, and it is difficult to explain these sound differences in words as the vocabulary available to describe sound is rather limited -even though the underlying sonic effects are real. Audiophiles often use words like 'warm', 'liquid', 'smooth' and 'midrange magic' to describe tube amplifiers' sound.

Some claim that the midrange reproduction is more extended and smoother with tube amplifiers, but that high frequencies are somewhat rolled off. Historically this was often the case due to limitations in capacitor performance. Modern audiophile grade tube amplifiers however, using modern and/or premium quality (and cost) capacitors can have frequency response that are essentially flat to octaves beyond the audio range, -3dB above 65kHz would be normal, above 85kHz is quite common

Similarly, some would characterise "tube sound" as bass response with less power and/or less definition (perhaps even "sloppy" bass or a bass boom with some speakers.) This again can be explained by many tube amplifiers having relatively high output impedance (Z out) compared to transistor designs, due to the combination of both higher device impedance itself and typically reduced feedback margins (more feedback results in a lower Z out).

So for example a hypothetical design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" midrange (due to reduced 2nd order harmonics due to greater reduction of distortion) but a "tighter" bass due lower output impedance. The speaker impedance divided by the Z out is sometimes referred to as the "damping factor" - the amplifier's ability to control the mechanical movement of the speaker.

In general terms the sound from a tube amplifier will typically have a softer attack and the bass frequencies will be more prominent giving a warmer and "less harsh" sound. Instrumentation such as pianos and vocals sound softer and "fatter" than with transistor amplifiers. But as noted the reasons for these effects are not simply and unavoidably related to the gain device type, today a good designer using either technology has to make synergistic design compromise choices. And the sonic differences are less stereotyped than they used to be as a result.

Harmonic content and distortion Triodes (and Mosfets) produce a monotonically decaying harmonic distortion spectrum.

Psychoacoustic effects include that the stronger, lower harmonic products tend to dominate and mask the sound of the weaker, higher harmonic products

Even order harmonics sound as musical chords (notably octaves), which subjectively makes the sound "richer". Odd order harmonics sound less pleasant. Aharmonic distortion is discordant and is often implicated in designs that sound "brash", "harsh", "brittle", etc.

Push Pull amplifiers use two nominally identical gain devices "back to back". One consequence of this is that all even order harmonic products cancel, leaving the - subjectively less musical, less "rich" - odd order products to dominate. The total (measured) harmonic distortion content is lowered, but subjectively the design may sound worse. A push-pull amplifier is said to have a symmetric (even and odd functions) transfer characteristic, and accordingly produces only odd harmonics.

A single-ended amplifier has an asymmetric transfer characteristic, and produces both even and odd harmonics. Ask the Doctors: Tube vs. Solid-State Harmonics — Universal Audio Webzine Volume cranked up in amp debate — Electronic Engineering Times{{cite conference ] region the distortion will be as described above, but while in class B the distortion will be SE like. Also, a class AB amplifier will have crossover distortion that will be typically inharmonic and thus sonically very undesirable indeed.

Another factor is that the distortion content of class A circuits (SE or PP) typically monotonically reduces as the signal level is reduced, asymptotic to zero during quiet passages of music. For this reason class A amplifiers are especially desired for classical and acoustic music etc. cf. class B and AB amplifiers, for which the amplitude of the crossover distortion is more or less constant, and thus the distortion relative to signal in fact increases as the music gets quieter. Class A amplifiers measure best at low power, class AB and B amplifiers measure best just below max rated power.

Loudspeakers present a reactive load to an amplifier (capacitance, inductance and resistance). This impedance may vary in value with signal frequency and amplitude. This variable loading affects the amplifiers performance both because the amplifier has finite output impedance (it cannot keep its output voltage perfectly constant when the speaker load varies) and because the phase of the speaker load can change the stability margin of the amplifier. The influence of the speaker impedance is different between valve amplifers and transistor amplifiers, principally because valve amplifiers normally use output transformers and they tend to use relatively low feedback.

Tube and transistor amplifier designs compared There has been considerable debate over the characteristics of tubes versus bipolar junction transistors.Some audiophiles have argued that the quadratic curve transconductance of tubes compared with the exponential transconductance of transistors is an important factor. This has not been proven.

Some audiophiles argue that devices are not as important as circuit topology. Triodes and MOSFETs have certain similarities in their transfer characteristics, whereas later forms of the tube, the tetrode and pentode, have quite different characteristics that are in some ways similar to the bipolar transistor. Despite this, eg MOSFET amplifier circuits typically do not reproduce tube sound any more than typical bipolar designs, due to the circuit topology differences between a typical tube design and a typical mosfet design. But there always exceptions: for example some very interesting designs such as the Zen series by Nelson Pass which can be found on the web.

Soft clipping Soft clipping is a very important aspect of tube sound especially for guitar amplifiers, although a HiFi amplifier should not normally ever be driven into clipping.

A tube amplifier will reproduce a wave relatively linearly to a point, and as the signal moves beyond the linear range of the tube (into overload), it distorts the signal with a smooth curve instead of a sudden, sharp-edged cutoff (or even ringing and/or lockup) as occurs with transistors. The harmonics added to the signal are of lower energy with soft clipping than hard clipping. However, soft clipping is not exclusive to tubes, it can be simulated in transistor circuits (below the point that real hard clipping would occur); see valve sound#Intentional creation of distortion. Note also that tube circuits often have huge headroom (overload) margins due to the high voltages they run from, so hard clipping is in reality very rare in a tube stage itself. However core saturation in the output transformer may be "designed in" to some guitar amplifiers when driven hard, and/or the tube biasing may be designed so that the tube passes from class AB1 to class AB2 and starts to draw grid current etc. (these effects are perhaps beyond the scope of this article)

Circuit design may also play an important role in the tube sound; tube circuits are often less complex and laid out differently. It is argued that simplicity is usually best, as the length and complexity can change the inductance and capacitance of a circuit.

A more complex circuit will have a more complex sonic / distortion characteristic. Minimalist DH-SEs for example typically have a dominant very simple harmonic distortion spectrum. Complex modern transistor designs often have low level but extremely complex harmonic distortion spectra.

Bandwidth Early tube amplifiers often had only limited bandwidth, in part due to the passive component technology available on a budget at the time, notably resistors. However modern / premium components make it easy to produce amplifiers that are essentially flat over the audio band, with -3db points at order of 6Hz to 70kHz, several octaves beyond the musical / audio range.

Negative feedback Tube amplifiers could not, and did not need to, use as much negative feedback (NFB) as transistor amplifiers due to the large phase shifts caused by the output transformers and their lower stage gains. While the absence of NFB slightly increases harmonic distortion, it avoids instability, as well as slew rate and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers.

Power supplies Early tube amplifiers usually used unregulated power supplies. This was due to the high cost associated with high-quality high-voltage power supplies. The typical anode supply was simply a rectifier, an inductor and a electronic filter. When the tube amplifier was operated at high volume, the power supply voltage would dip, reducing power output and causing signal modulation. This dipping effect is known as "sag", which may be preferable to some electric guitarists (almost invariably using a class AB1 amplifier). Note that for a class A stage the average current does NOT sag, it is constant.

In contrast, modern amplifiers often use high-quality, well-regulated power supplies. In theory, the output voltage remains constant, but in reality it never does - not least due to resistive losses in the cabling from the power supply to the gain stage. This problem is proportionately much worse in transistor amplifiers because they operate at low voltage and high current, whereas tube voltage amplification stages operate at low currents and high voltages. Ohmic losses are a function of current through resistance.

Push-pull amplifiers A Class A push-pull amplifier produces exceptionally low distortion for any given level of applied feedback, and also cancels the flux in the transformer cores, so this topology is seen by some as the ultimate "engineering" approach to the tube hi-fi amplifier for use with normal speakers. Output power of 10W is possible using standard tubes, and up to 25W using "reasonable" extreme tubes.

The majority of commercial HiFi amplifier designs are Class AB, in order to deliver greater Electric power andelectrical efficiency, typically 12 - 25 watts upwards. Such designs will invariably use at least some NFB.

Class AB push-pull topology is nearly universally used in tube amps for electric guitar applications.Whereas audiophile amps are primarily concerned with avoiding distortion, a guitar amp embraces it. When driven to their respective limits, tubes and transistors distort quite differently. Tubes clip more softly than transistors, allowing higher levels of distortion (which is sometimes desired by the guitarist) whilst still being able to distinguish the harmonies of a chord. This is because the soft profile of the tube amplifier's distortion means that the intermodulation products of the distortion are generally more closely related to the harmonies of the chord.

Single-Ended Triode (SET) amplifiers SET amplifiers typically measure very badly - they have low output power, are inefficient, have poor damping factors and high measured distortions.

The triode, despite being the oldest signal amplification device, also has the most linear transfer characteristic, and thus requires little or no negative feedback for acceptable distortion performance.NFB is used in most post 1950s amplifiers and although it usually reduces the measured distortion level, it results in an unpleasant combination of harmonics to some ears.

Some audiophiles argue that measured sound performance is a poor indicator of real world sound performance. In the 1970s, designers started producing transistor amps with higher open loop gain to support a greater value of negative feedback. These amps produced near perfect measured results but some listeners believed that these amplifiers sounded "cold" or "dull". In the following years, amplifiers were built with modest gain but good open loop linearity, deployed with only minimal levels of NFB.

Despite their linearity, SETs do distort, with a unique distortion pattern of simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic distortion. The result is like adding the same tone one octave higher. The added tone is usually lower, at about 5% or less in a no feedback amp. Some argue that this "distortion" can actually enhance the music, making it sound somewhat richer.

SETs usually only produce about 5 to 10 watts or less; the most expensive amp in existence, the Wavac SH-833 monoblock SETs (which cost about US$350,000) produces about 150 watts using an 833A tube. Large amounts of power are not necessary in amplifiers, as only a few watts are required to drive most audiophile speakers to a SPL of nearly 100 dB at 1 m. Their low power also makes them ideal for use as preamps.

Intentional creation of distortion Tube sound from transistor amplifiers Some individual characteristics of the tube sound, such as the Distortion synthesis on overdrive, are straightforward to produce in a transistor circuit or digital filter. For more complete simulations, engineers have been successful in developing transistor amplifiers that produce a sound quality very similar to the tube sound. Usually this involves using a circuit topology similar to that used in tube amplifiers.

In 1982, Tom Scholz, a graduate of MIT and a member of Boston (band), introduced the Rockman (music), which used bipolar transistors, but achieved a distorted sound adopted by many well known musicians. Advanced digital signal processing offers the possibility to simulate tube sound. Computer algorithms are currently available that transform digital sound from a CD or other digital source into a distorted digital sound signal.

Using modern passive components, and modern sources, whether digital or analogue, and wide bandloudspeakers, it is possible to have tube amplifiers with the characteristic wide bandwidth and "fast"sound of modern transistor amplifiers, including using push-pull circuits, class AB, and feedback. Some enthusiasts have built amplifiers using transistors and MOSFETs that operate in class A, including single ended, and these often have the "tube sound" .

Tube/transistor "HYBRID" amplifiers Tubes are often used to impart characteristics that many people find audibly pleasant to solid state amplifiers, such as Musical Fidelity's use of Nuvistors, tiny triode tubes, to control large bi-polar transistors in their NuVista 300 power amp. In America, Moscode and Studio Electric use this method, but use MOSFET transistors for power, rather than bi-polar. Pathos, an Italian company, has developed an entire line of hybrid amplifiers.

To demonstrate one aspect of this effect, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The slow response of the light bulb's conductance (which varies according to temperature) can thus be used to moderate the sound and attain a tube-like "soft limiting" of the output, though other aspects of "the tube sound" would not be duplicated in this exercise.

Tube sound enthusiasts Some enthusiasts consider that "pure" tube amplifiers should not use anything except tubes as active devices. Others, in contrast, will use tubes for the audio circuit, but will accept the use of semiconductor gain devices in the power supply or as constant current sources. Other schisms concern the use of triodes vs. tetrodes and pentodes, and the use of directly heated tubes vs. indirectly heated tubes.

Many of the explanations relate to the circuit topologies pioneered using tubes, and traditionally associated with them ever since, regardless of whether they are built using tubes today, notably the directly heated single-ended triode amplifier circuit, which operates in Class A amplifier and often has no negative feedback; this topology is a classic source of the tube sound.

See also

Notes References

Barbour, Eric. The Cool Sound of Tubes in IEEE Spectrum Online.
Hamm, Russell O. (September 14, 1972). "Tubes vs. Transistors: Is There An Audible Difference?". Presented at the 43rd convention of the Audio Engineering Society, New York.
Reisch, George. Scientists vs Audiophiles 1999 in Stereophile, March, 1999.